Composite bearing material with polymer filled metal matrix interlayer of distinct metal particle sizes and method of making same

ABSTRACT

The present invention relates to a composite bearing material comprising three layers: (1) a metal backing, (2) a porous metal interlayer bonded to the metal backing which includes bronze or other copper alloy particles from two distinct particle size ranges and (3) a PTFE based composition intermixed with and overlying the porous metal interlayer. In accordance with the method of the present invention, the porous metal interlayer substantially comprises a relatively non-homogeneous mixture of particles from the two distinct particle size ranges, wherein a substantial portion of the relatively fine powder is segregated adjacent the metal backing. The present invention includes a significant thickness of the PTFE based composition applied to and remaining above the surface of the porous metal backing.

This application is a continuation of application Ser. No. 607,863 filedApr. 30, 1984, now abandoned.

CROSS-REFERENCE TO RELATED APPLICATION

The subject matter of this application is related to the subject matterdisclosed in application Ser. No. 605,037, entitled "Method of Making aPTFE Based Tape Suitable for Impregnation into Porous Metal Matrix" nowU.S. Pat. No. 4,615,854, with the inventors being noted as George C.Pratt and Michael C. Montpetit, filed Apr. 30, 1984, and assigned to thesame assignee.

BACKGROUND OF THE INVENTION

This invention generally relates to an improved bearing material andrelated method, and more particularly to an improved composite bearingmaterial which includeas polytetrafluoroethylene (PTFE).

Dry bearing materials, i.e., those which will operate as a bearingwithout the benefit of a lubricant applied to the bearing surface, arewell known and may generally be considered to be of three main types:(1) homogeneous materials which may be molded or pressed and machined,any surface of which can be the bearing surface; (2) non-homogeneousmaterials generally taking the form of a backing material and a bearinglayer, in which the bearing layer is a dry bearing composition; and (3)non-homogeneous materials generally taking the form of a backingmaterial and an impregnated interlayer, and having a relatively thinsurface layer over the interlayer, which surface layer generally cannotbe machined without seriously adversely affecting the bearing capabilityof the material because of exposure of the interlayer. The presentinvention may generally be considered to be a composite material of thethird general type referred to above, except that the surface layer isof significant thickness such that it can be machined without seriouslyaffecting bearing capability.

Numerous situations arise wherein a bearing material is required toprovide good wear resistance and low friction under conditions ofmoderate load and temperature while operating in a substantially dry ornon-lubricated environment. A variety of composite bearing materials andplastic materials which incorporate desirable bearing characteristicsand are of good wear resistance under the foregoing operating conditionshave heretofore been used or proposed for use as bearing materials. Ofthe various dry bearing materials known, those based onpolytetrafluoroethylene (PTFE) have perhaps received the most widespreaduse for this purpose. While dry bearing materials incorporating PTFEhave been found to provide satisfactory performance under many differentbearing operating conditions, an inherent disadvantage of suchmaterials, when they are of the third general type described above, is,as noted above, the difficulty encountered in the finish machiningthereof into bearings of the desired configuration and size. It isdesirable therefore to retain the advantages of the third general typeof bearing material, while at the same time providing the product with adegree of machinability. Among such advantages of this third generaltype of bearing material are: (1) a good bond between the lining and thebacking (the porous interlayer being metallurgically bonded to thebacking, and the polymer being keyed into the porous interlayer)--(Thisis particularly important in the case of PTFE which is difficult to bondto a backing. Composites of the second general type referred to above inwhich the lining is PTFE based tend to have a weak bond.); (2)dimensional stability (shared with the second general type referred toabove when the dry bearing composition lining is thin); and (3) highthermal conductivity (shared with the second general type referred toabove when the dry bearing composition lining is thin).

Dry bearing materials consisting of a metal backing and a porous metalinterlayer impregnated with PTFE are well known. Generally, the PTFE isapplied to the surface of the porous interlayer as a thick pasteobtained by the coagulation of a dispersion of micron size PTFEparticles in water. Sufficient water is retained in the paste to make itamenable to roll impregnation into the porous interlayer. Metallic ornon-metallic fillers may be incorporated by mixing them in powder formwith the PTFE before coagulation or at some stage during the preparationof the paste.

A composite bearing material of the third general type referred to aboveis normally produced as continuous strip, starting with a coil of steel,(which may be plated), to the surface of which bronze power is sinteredto from a porous bronze layer. PTFE paste, with or without filler(s), isgeneraly applied to the surface of the porous bronze with a spoon. Thestrip then passes under a roller which spreads the paste over thesurface of the bronze as a reasonably uniform layer. It then passesthrough a rolling mill which forces the paste into the porous bronze,leaving little or no paste above the bronze. The strip is then heated toa temperature in excess of 327° C. during which process the remainingwater is driven off and the PTFE is sintered.

The method of applying the PTFE described above has severaldisadvantages, First, the paste is typicaly applied in discretespoonfuls making it difficult to achieve a perfectly uniformdistribution over the bronze surface. Second, the processs is typicallylabor intensive, since an operator with a spoon is often solely occupiedwith spooning the paste on to the strip. It has proved somewhatdifficult to mechanize this operation.

There is a third disadvantage which applies to certain forms of theproduct. In a common form of the product, a single filler, in the formof lead powder, is incorporated into the PTFE. During roll impregnationof the PTFE/lead paste into the porous bronze layer, only a very thinlayer of paste, usually less than about 30 microns (1.2×10⁻³ inches)thick is typically left above the surface of the bronze. The thicknessof this surface layer is substantially unchanged during sintering of thePTFE. The surface layer has very low wear resistance, and in service thesurface of the composite wears rapidly until the bronze surface isexposed in spots in the area of rubbing. Wear rate then falls to a lowvalue. As wear proceeds, the proportion of bronze exposed in the area ofrubbing gradually increases. When the proportion reaches approximately10%, wear rate begins to increase and bronze is exposed at an increasingrate. Wear rate increases rapidly and the useful dry bearing life of thecomposite is near an end.

In other forms of the product, more than one filter is incorporated intothe PTFE. If the fillers are appropriately chosen, the surface layer maybe made to possess a high degree of wear resistance. A low wear rate isnot dependent on the exposure of the impregnated bronze surface in thearea of rubbing. In such forms of the product, it is an advantage tohave a thick layer of PTFE above the bronze, since the dry bearing lifeof the commposite is thereby increased.

The third disadvantage referred to above of applying the PTFE as a pasteby spoon to the bronze surface is that it has proved difficult by thisprocess to leave a thick layer of PTFE above the bronze duringimpregnation. If the consistency of the paste is sufficiently soft forspooning and spreading evenly over the surface of the bronze, it isoften so soft that some excess PTFE is often squeezed off the edges ofthe strip during impregnation and is not retained above the bronze as athick surface layer. It has therefore proved difficult to achieve theincreased dry bearing life theoretically made possible by theincorporation of appropriate fillers in the PTFE.

Development of a technique for creating a tape of PTFE incorporatingcontrolled amounts of filler(s), suitable for roll impregnation into aporous metal matrix or interlayer, has been necessitated by the peculiarnature of PTFE. Although PTFE is classed by polymer chemists as athermoplastic, it does not melt like other typical thermoplastics. Atits transition temperature of 327° C.,, it changes to a rubber-likestate generally unsuitable for melt processing. Tape of the typesuitable for impregnation into a porous matrix cannot thereforegenerally be produced by extrusion like other thermoplastic polymers,such as nylon based materials, hexafluoropropylene based materials, orthe like. The common method of producing sintered PTFE tape is to pressand sinter a cylindrical block of the polymer, with or without theincorporation of fillers, and to skive off a tape from the surface ofthe cylinder with a knife. Unsintered PTFE tape can also be produced,and is commonly used for sealing threaded joints. Also, the use of aconveyor and compressing roller system including a belt made of afiltering or permeable material (such as felt), and the application of avacuum to both the belt and to a PTFE based composition (which mayinclude fillers) thereon, to produce PTFE based sheeting or tape isknown. However, these forms of PTFE tape are not believed to be totallysuitable for roll impregnation into porous metal sinter, since in someinstances the PTFE is sufficiently strong even above its transitiontemperature to compact the porous metal interlayer instead ofcontinously and fully impregnating into it, as is often desired.Successful impregnation may under certain circumstances be obtained, butmay require the application of pressure at elevated temperature fortimes often impracticable for a reasonable roll impregnation stripprocess required to operate at reasonably economic speeds.

It is accordingly a principal object of the present invention to providea improved composite bearing material which posseses excellent drybearing characteristics and has good wear resistance in the presence oflimited or no lubrication when operating under moderate load andtemperature conditions.

Another object of the present invention is to provide an improvedcomposite bearing material which incorporates controlled proportions ofselected filler materials which enhance the dry bearing characteristicsof the material such that improved wear resistance of the material isobtained.

Still another object of the present invention is to provide an improvedcomposite bearing material having a surface layer of significantthickness which may be machined without seriously affecting bearingcapability.

SUMMARY OF THE INVENTION

The foregoing objects and other advatages of the present invention areachieved by providing a composite bearing material comprising threelayers: (1) a metal backing, which will normally be a low carbon steeland which may be plated such as with nickel plating or the like, (2) aporous metal interlayer, which may comprise bronze or other copperalloys, on said metal backing, and (3) a PTFE based compositionformulated to have good wear resistance. The pores in the interlayer aresubstantially filled with the PTFE based composition, which is appliedin such a way as to leave the desired thickness of PTFE basedcomposition above the porous metal interlayer to form a surface layer.The main function of the interlayer is to key the PTFE based compositionand surface layer to the metal backing. In accordance with the presentinvention, the power used to form the porous metal interlayer issubstantially comprised of particles from two distinct particle sizeranges obtained by blending relatively fine powder particles withrelatively coarse powder paticles. In accordance with the method of thepresent invention, in forming a porous metal interlayer on a metalbacking, with the porous metal interlayer being suitable forimpregnation with a PTFE based composition to form a composite bearingmaterial, the porous metal interlayer is appilied to the metal backingas a relatively homogeneous mixture of metal powder substantiallycomprising particles from the two distinct particle size ranges, namely,the relatively fine powder range and the relatively coarse powder range.During and after being applied to the metal backing and prior to thecompletion of sintering, a substantial portion of the relatively finepowder from the relatively homogeneous mixture of metal powder issegregated adjacent the metal backing so that a relativelynon-homogeneous mixture of metal powder is formed on the metal backing.The two particle size ranges are selected so as to provide for suchsegregation. In addition, the PTFE based composition which isimpregnated into the porous metal interlayer in accordance with thepresent invention comprises from about 2% to about 10% by volume of atleast one one material selected from the group consisting of tin bronzeand other copper alloys, as well as mixtures thereof; from about 5% toabout 30% by volume of at least one material selected from the groupconsisting of metallic lead, metallic cadmium, an oxide of lead, and anoxide of cadmium, as well as mixtures thereof; from about 5% to about30% by volume of at least one material selected from the groupconsisting of natural graphite and artificial graphite, as well asmixtures thereof; with the remainder comprising polytetrafluoroethylene.As will be described further hereinbelow, the present invention enablesa significant thickness of the PTFE based composition to be applied andremain above the surface of the porous metal backing. The presentinvention is especially suited to provide low friction and a high degreeof dry wear resistance. The low friction derives from the PTFE matrix,while the good wear resistance derives from the combination of fillersincorporated into the PTFE.

Additional benefits and advantages of the present invention will becomeapparent upon a reading of the detailed description of the preferredembodiment taken in conjunction with the accompanying examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a cross-section of bearingmaterial made in accordance with the present invention;

FIG. 2 is a schematic representation of a mixing apparatus used inconnection with a preferred method of makind the product of the presentinvention;

FIG. 3 is a schematic representation of a hopper, roller system, andwater-removal system also used in connection with a preferred method ofmaking the product of the present invention; and

FIG. 4 is a schematic representation of a further roller system and ovensystem also used in connection with a preferred method of making theproduct of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein the showings are for the purposeof illustrating a preferred embodiment of the present invention and arenot for the purpose of limiting the invention, FIG. 1 shows a schematiccross-sectional view of bearing material 10. The bearing material 10generally comprises three layers: (1) a metal backing 12, (2) a porousmetal interlayer 14, and (3) a PTFE based composition 16. The metalbacking 12 will normally be low carbon steel, although other suitablebacking materials should also be useable. In addition, in order toenhance adhesion of the porous metal interlayer 14 to the metal backing12, the metal backing 12 may be plated with a layer 18 of nickel platingor other conventional plating materials.

The porous metal interlayer 14 may consist of bronze or other copperalloys such as tin bronze or leaded bronze. The porous metal interlayeror porous metal matrix is produced by spreading the copper alloy powderonto the metal backing and heating in a conventional manner in anon-oxidizing atmosphere to a temperature at which sintering of thepowder particles to one another and to the metal backing takes place.For example, a temperature of 850 degrees C. in a nitrogen-hydrogenatmosphere is suitable for sintering tin bronze particles. The powderused to form the porous metal interlayer consists of two distinct sizeranges of particles obtained by blending relatively fine powderparticles with relatively coarse powder particles to form a relativelyhomogeneous mixture. During and after spreading of the powder layer andbefore the completion of sintering, a substantial portion of therelatively fine particles segregate down to the metal backing surfacewhere they promote formation of the bond between the interlayer and themetal backing. Hence a relatively non-homogeneous mixture of metalpowder is formed on the metal backing. The surface activity of the fineparticles during sintering is greater than that of the coarse particles,and hence, a stronger bond is established in a given time than if thefine particles were not present. Segregation of the fine particles tothe steel surface leaves an open structure substantially of coarseparticles in the upper surface of the interlayer ideally suited toimpregnation with the PTFE based composition. Such segregation isrepresented schematically in FIG. 1 where the large particles 20represent coarse bronze powder or the like and the small particles 22represent fine bronze powder or the like. The thickness of the porousmetal interlayer after sintering is preferably from about 0.25 mm (0.010inches) to about 0.40 mm (0.016 inches). It has been found thatrelatively coarse powders of the order of from about -60 to about +150mesh (Tyler units or the like), preferably of the order of about -60mesh about +100 mesh, mixed with relatively fine powders of the order ofabout -300 mesh give good results. (A minus sign prior to a meshdesignation indicates that the powder will go through the stated mesh,while a plus sign indicates that the powder will be retained on themesh. For example, -300 indicates particles of 300 mesh size andsmaller.) In addition, it has been found that the addition of wax duringmixing of the two powder fractions prior to transfer of the mixed bronzepowders to the hopper used to hold and distribute the powder on themetal backing, prevents premature segregation of the two size fractionsof powder without affecting spreadability. The addition of about 0.5% byweight of "Acrawax" to the bronze powder by tumbling has been found togive satisfactory results. "Acrawax" is a synthetic amide waxmanufactured by Glyco Chemical Corporation, and chemically comprisesethylene di-stearamide. In addition, zinc stearate may also be usable.This wax material volatilizes during sintering.

With regard to the PTFE based composition, it has been found possible tooptimize wear resistance of the PTFE composition by the incorporation ofthree distinct types of filer materials. The fillers are preferably: (1)tin bronze, (2) metallic lead, and (3) natural graphite. (These arerepresented schematically in FIG. 1 as particles 24, 26, and 28respectively.) Other copper alloys may be substituted for the tinbronze; metallic cadmium, an oxide of lead, or an oxide of cadmium maybe substituted for the lead; and artificial graphite may be substitutedfor the natural graphite. However, some loss of dry wear resistace willresult from such substitutions. Mixtures of each of these fillers arealso believed to be usable. The filler component which is at least onematerial selected from the group consisting of tin bronze and othercopper alloys, as well as mixtures thereof, should preferably be about-300 mesh powder and should generally be present in the range of fromabout 2% to about 10% by volume of te PTFE based composition, with fromabout 4% to about 6% by volume being preferred, and with about 5% byvolume providing optimum results. The filler component which is at leastone material selected from the group consisting of metallic lead,metallic cadmium, an oxide of lead, and an oxide of cadmium, as well asmixtures thereof, should preferably be about -200 mesh powder and shouldgenerally be present in the range of from about 5% to about 30% byvolume of the PTFE based composition, with from about 15% to about 30%by volume being preferred, and with about 20% by volume providingoptimum results. The filler component which is at least one materialselected from the group consisting of natural graphite and artificialgraphite, as well as mixture thereof, should preferably be about -325mesh powder and should generally be present in the range of from about5% to about 30% by volume of the PTFE based composition, with from about10% to about 25% by volume being preferred, and with about 10% by volumeproviding optimum results. PTFE makes up the remainder of the liningcomposition.

In order to further describe the illustrate the present invention, thefollowing examples are provided. It will be understood that theseexamples are provided for illustrative purposes and are not intended tobe limiting of the scope of the invention as herein described and as setforth in the subjoined claims.

In Table I hereinbelow, the effect of incorporating various proportionsof relatively fine bronze powder (-300 mesh in this example) intorelatively coarse bronze powder (-60 to +100 mesh in this example) onboth the bond strength to the steel substrate and on the porosity of thebronze layer is shown. Needless to say, the results given are aftersintering. As shown in this table, the optimum percentage by weight ofabout -300 mesh powder giving a high level of both bond and porosity isabout 20%. In general, the relatively fine powder preferably comprisesfrom about 5% to about 30% by weight of the total powder used to producethe porous metal interlayer. Percent bond was metallographicallymeasured by conventional techniques by determining the amount of bronzein contact with the steel surface, which was nickel plated. Percentporosity was methallographically measured by conventional techniquesusing an image analyzer.

                  TABLE I                                                         ______________________________________                                                                % porosity                                            % by weight   % bond    in the sintered                                       fine powder   to the steel                                                                            bronze layer                                          ______________________________________                                         0            23.6      47.3                                                  10            41.4      42.6                                                  20            87.3      43.8                                                  30            83.7      32.8                                                  40            98.8      19.6                                                  50            94.5      20.8                                                  ______________________________________                                    

Table II hereinbelow presents data on the wear of PTFE basedcompositions made in accordance with the present invention after rubbingagainst a 2.125 inch diameter steel shaft. A rotational speed of 980rpm, a load of 11 lbs., and a test duration of 16 hours were used. (ThePTFE based compositions were in sintered block form for these tests.)

                  TABLE II                                                        ______________________________________                                        Composition % by                                                              Volume Filler -Remainder PTFE                                                 Tin Bronze                                                                            Lead       Natural Graphite                                                                           Wear × 10.sup.-3                        (-300)  (-200)     (-325)       Cubic Inches                                  ______________________________________                                        2.5     10          0           0.17                                          2.5     10         10           0.27                                          2.5     10         20           0.20                                          5       10          0           0.35                                          5       10         10           0.22                                          5       10         20           0.16                                          10      10          0           0.57                                          10      10         10           0.55                                          10      10         20           0.40                                          2.5     20          0           0.13                                          2.5     20         10           0.21                                          2.5     20         20           0.19                                          5       20          0           0.13                                          5       20         10            0.10*                                        5       20         20           0.28                                          10      20          0           0.21                                          10      20         10           0.20                                          10      20         20           0.32                                          ______________________________________                                         *best result                                                             

The PTFE based compositions used with the present invention mayconveniently be applied to the porous metal interlayer usingconventional methods such as in the form of a paste incorporating asuitable proportion of a liquid such as water with the PTFE and fillers.The paste is impregnated into the porosity of the metal interlayer forinstance by conventional rolling techniques, the rolling conditionsbeing chosen such as to leave a layer from about 0.05 mm (0.002 inches)to about 0.5 mm (0.020 inches) thick above the surface of theinterlayer. The PTFE based composition is then sintered in aconventional manner at a temperature above 327 degrees C., andpreferably below 400 degrees C., during which process the liquidevaporates and a firm bearing lining is created.

In addition to the above-described paste technique, a preferred methodof applying the PTFE based composition of the present invention isclaimed in application Ser. No. 605,037, entitled "Method of Making aPTFE Based Tape Suitable for Impregnation into a Porous Metal Matrix",with the inventors being noted as George C. Pratt and Michael C.Montpetit, filed on the same day as this application, and assigned tothe same assignee, now U.S. Pat. No. 4,615,854. That application, whichis hereby incorporated by reference herein, relates to a process forapplying PTFE to the surface of metal backed porous bronze strip or thelike which is believed to overcome some of the disadvantages ofconventional rolling techniques described hereinabove. The processpermits the application of a relatively uniform layer of PTFE; it is notlabor intensive; and full impregnation of the porous metal interlayermay be obtained while leaving a relatively thick layer of PTFE above theporous metal interlayer. The process is primarily intended for applyingPTFE incorporating more than one filler combining to endow the surfacelayer with a high degree of dry wear resistance. The process is howeverapplicable also to PTFE with less than two fillers.

The process generally consists of the following stages: mixing thefillers into an aqueous dispersion of PTFE and adding a water thickenerand coagulant, transferring the resultant paste (having a consistencyreminiscent of porridge) to a machine in which it is sandwiched betweena continuously moving band of supporting material and a band of high wetstrength filter paper, compacting or compressing the sandwich andremoving excess water by rolling, transferring the sandwich to a movingbelt comprising a gauze-like material, removing some of the remainingwater through the filter paper and the gauze belt by suction, furthercompacting or compressing the sandwich, removing the layer of filterpaper, coiling the supporting material with the PTFE based materialadhering to one side, and roll impregnating the PTFE based material intoa metal backed porous metal interlayer and peeling off the layer ofsupporting material.

The process is illustrated in FIGS. 2, 3, and 4, and the stages in thepreferred embodiment are as follows. For example, 810 ml (or 990 grams)of an aqueous dispersion of PTFE (E. I. DuPont de Nemours & Co.,"DuPont", T35) containing 33% by weight PTFE are poured into a container30 and stirred by a paddle 32 driven by an electric motor 34. 520 gramsof -200 mesh metallic lead powder, 50 grams of -325 mesh naturalgraphite powder (technical grade), and 100 grams of -300 mesh in tinbronze powder are added to the PTFE dispersion while stirring. (Theabove amounts of lead, graphite, and tin bronze correspond to theoptimum amounts referred to above, namely about 20%, 10%, and 5% byvolume, respectively, of the final PTFE based cmposition.) 33 grams of alatex-based polymeric water thickener (Rohm and Hass TT615), 50 ml ofammonium hydroxide (28-30% NH₃, normality of 15, reagent grade), and 100ml of a 250 gram/liter solution of aluminum nitrate (reagent grade) areadded, and stirring is continued until the mixture attains theconsistency of a thick paste. The aluminum nitrate function as ascoagulant, while the ammonium hydroxide appears to act as a catalyst.

Referring now to FIG. 3, the paste is transferred to a hopper 36, onewall of which supports a moving band of supporting material 38 (such asMylar^(R) biaxially oriented polyethylene terephthalate made by DuPont),and another wall of which supports a moving band of high wet strengthfilter paper 40 (such as Eaton Dyteman Grade 909/20). The supportingmaterial and the filter paper are fed from rolls above the hopper 42 and44, respectively. The hopper has adjustable side walls 46.

The hopper is sited immediately above a pair of rolls 48 and the pasteis caried into the roll gap sandwiched between the supporting materialand the filter paper. The PTFE based material is compressed to athickness of about 1.25 mm (0.050 inches). The sandwich is then carriedon a gauze belt 50 (such as semi-rigid polyester sieve-type gauzeavailable from Sagent-Welch as polyester monofilament sieve clothS74616P), which is moved b rollers 52, across the inlet to a vacuum pump54 which removes some of the water fom the PTFE based material bysuction (15 inches of mercury (dynamic vacuum) drawn over a 14 squareinch area in this example) through the filter paper and the gauze. Thesandwich then passes between a second pair of rolls 56 where the PTFEbased material is compacted to a final thickness of about 0.25-0.50 mm(0.010-0.020 inches, about 0.015 inches is preferred). The filter paperis then peeled from the PTFE based material, and the supporting materialwith a tape of PTFE based material adhering to one side is coiled into aroll 58. The water content of the PTFE based tape is approximately 20%.

Referring now to FIG. 4, the coil 58 of PTFE based tape interleaved withsupporting material from the preceding step is then mounted on the inputside of a roll impregnating mill 60. As a steel backed porous bronzestrip 62 (the bronze interlayer is about 0.30 mm thick) moves throughthe mill, (the steel is nickel-plated in a preferred embodiment), theinterleaved tape is fed into the roll gap, where the PTFE based materialis forced into the pores of the bronze. The supporting material ispeeled off the surfaace of the PTFE based material and coiled into aroll 64. The mill gap is controlled so as to leave the desired surfacelayer of PTFE based material, approximately 0.15 mm (0.006 inches) thickabove the bronze. The composite strip then passes through an oven 66 inwhich the PTFE based material is sintered in a conventional manner at atemperature of 350° C. (One skilled in the art will recognize that thevarious apparatus of FIGS. 3 and 4 could be combined to make acontinuous operation without an intermediate roll-up step at 58.) Also,materials other than the specific chemicals and/or other materialsreferred to in connection with the above tape process may also beusable.

In the tape process described above, the extent to which the PTFE basedmaterial is "mechanically worked" is kept to a minimum and a smallproportion of liquid, water in the above example, is retained in thePTFE based tape. The tape is weak and uses the liquid as a lubricant,and for this reason has to be carried into the impregnating mill on itssupporting material backing. Because of its lack of strength, however,it can be forced into the pores of the porous metal interlayer withoutcompacting that layer and closing up the pores.

Bearing components such as bushings and thrust washers can then beformed from the composite bearing material by conventional techniques.Such components will have excellent wear resistance in dry bearingapplications, and may also be used with advantage in certain lubricatedbearing applications.

While it will be apparent that the invention herein disclosed is wellcalculated to achieve the benefits and advantages as hereinabove setforth, it will be appreciated that the invention is susceptible tomodification, variation, and change without departing from the spiritthereof.

What is claimed is:
 1. A composite bearing material suitable for bothdry and lubricated bearing applications comprising:(a) a metal backing;(b) a porous metal interlayer on said metal backing, said porous metalinterlayer consisting essentially of a relatively non-homogeneousmixture of particles form two distinct particle size ranges including arelatively fine powder range of about -300 mesh and less and arelatively coarse powder range form about -60 to +150 mesh, saidrelatively fine powder range comprising from about 5% to about 30% byweight of the total powder constituting said porous metal interlayer andwherein a substantial portion of said relatively fine powder issegregated from the particles in said coarse powder range to theinnermost surface of said porous metal interlayer adjacent said metalbacking and bonded to said metal backing and certain of said particleswithin said relatively coarse range, said particles from said relativelycoarse range being bonded to one another and forming a open structure ofnumerous pores throughout at least the outermost surface of said porousmetal interlayer; and (c) a polytetrafluoroethylene based compositionsubstantially filling the pores of said porous metal interlayer andforming a surface layer on the bearing material, saidpolytetrafluoroethylene based composition comprising from about 2% toabout 10% by volume of at least one material selected from the groupconsisting of tin bronze and other copper alloys, as well as mixturesthereof; from about 5% to about 30% by volume of at least one materialselected from the group consisting of metallic lead, metallic cadmium,an oxide of lead, and an oxide of cadmium, as well as mixtures thereof;from about 5% to about 30% by volume of at least one material selectedfrom the group consisting of natural graphite and artifical graphite, aswell as mixtures thereof; and the remainder comprisingpolytetrafluoroethylene.
 2. A composite bearing material as defined inclaim 1 in which said metal backing comprises steel.
 3. A compositebearing material as defined in claim 1 in which said metal backingincludes a layer of plating thereon.
 4. A composite bearing material asdefined in claim 3 in which said plating comprises nickel plating.
 5. Acomposite bearing material as defined as in claim 1 in which said porousmetal interlayer comprises bronze or other copper alloys.
 6. A compositebearing material as defined in claim 1 in which said porous metalinterlayer comprises tin bronze or leaded bronze.
 7. A composite bearingmaterial as defined in claim 1 in which said relatively fine powderrange comprises about 20% by weight of the powder used to produce saidporous metal interlayer.
 8. A composite bearing material as defined inclaim 1 in which said polytetrafluoroethylene based compositioncomprises from about 4% to about 6% by volume of at least one materialselected from the group consisting of tin bronze and other copperalloys, as well as mixtures thereof; from about 15% to about 3% byvolume of at least one material selected from the group consisting ofmetallic lead, metallic cadmium, an oxide of lead, and an oxide ofcadmium, as well as mixtures thereof; from about 10% to about 25% byvolume of at least one material selected from the group consisting ofnatural graphite and artifical graphite, as well as mixtures thereof;and the remainder comprises polytetrafluoroethylene.
 9. A compositebearing material as defined in claim 1 in which saidpolytetrafluoroethylene based composition comprises about 5% by volumeof at least one material selected from the group consisting of tinbronze and other copper alloys, as well as mixtures thereof; about 20%by volume of at least one material selected from the group consisting ofmetallic lead, metallic cadmium, an oxide of lead, and an oxide ofcadmium, as well as mixtures thereof; about 10% by volume of at leastone material selected from the group consisting of natural graphite andartificial graphite, as well as mixtures thereof; and the remaindercomprises polytetrafluoroethylene.
 10. A composite bearing material asdefined in claim 1 in which said polytetrafluoroethylene basedcomposition has a thickness above the porous metal interlayer of fromabout 0.05 mm to about 0.5 mm.
 11. A composite bearing material asdefined in claim 1 in which said porous metal interlayer has a thicknessof from about 0.25 mm to about 0.40 mm.
 12. A method of making acomposite bearing material including the steps of:forming a porous metalinterlayer on a metal backing by applying to said metal backing arelatively homogeneous mixture of metal powder substantially comprisingparticles from two distinct particle size ranges, namely a relativelyfine powder range and a relatively coarse powder range, selecting saidtwo particle size ranges to provide for segregation of a substantialportion of said relatively fine powder from said relatively homogeneousmixture of metal powder such that the relatively fine powder shallconsist essentially of particles of -300 mesh size and less, and saidrelatively coarse powder shall consist essentially of particles rangingfrom about -60 meash size to about +150 mesh size, said relatively finepowder range comprising from about 5% to about 30% by weight of thetotal powder used to produce said porous metal interlayer, spreadingsaid relatively homogeneous mixture of metal powder onto said metalbacking and thereby causing a substantial portion of said relativelyfine powder to segregate to the innermost surface of said porous metalinterlayer adjacent said metal backing prior to the completion ofsintering to form a relatively non-homogeneous mixture of metal powderon said metal backing, sintering said metal powder on said metalbacking, impregnating and substantially completely filling said porousmetal interlayer with a polytetrafluoroethylene based composition, andthereafter sintering said filled porous metal interlayer at atemperature of at least about 350° C.